1. Field of the Invention
The present invention relates to a method of modifying a transparent conductive oxide film that can be used for a photovoltaic device or the like, and a method of manufacturing a photovoltaic device using such a modifying method.
2. Description of the Background
A transparent conductive oxide film, which is typically made of tin oxide, indium tin oxide or the like, is widely used to make a display element such as a liquid crystal display or a photovoltaic device such as a solar cell or an optical sensor.
In the field of photovoltaic devices, various strict performance conditions are required for such a transparent conductive oxide film in relation to electric characteristics, optical characteristics, and morphological characteristics of the film surface.
FIG. 12 is a sectional view showing an exemplary element structure of a photovoltaic device. Referring to FIG. 12, a transparent conductive oxide film 122 of tin oxide or indium tin oxide is formed on a transparent substrate 121 of glass or quartz. A photoelectric conversion layer 123 carrying out a photoelectric conversion function of the photovoltaic device is formed on the transparent conductive oxide film 122. This photoelectric conversion layer 123 is formed by a laminate of a p-type semiconductor layer 123p, an i-type semiconductor layer 123i and an n-type semiconductor layer 123n, which are parallel to the film surface. An electrode film 124 is formed on the photoelectric conversion layer 123.
Light 125, which is incident upon this photovoltaic device successively through the substrate 121 and the transparent conductive oxide film 122, is absorbed by the photoelectric conversion layer 123, to be extracted to the exterior as optical carriers consisting of electrons and holes from the electrode film 124 and the transparent conductive oxide film 122 respectively.
In such a photovoltaic device, a texture is formed on a surface 122a of the transparent conductive oxide film 122 as shown in FIG. 12. The transparent conductive oxide film 122 is made of tin oxide or indium tin oxide, which is structured as an aggregate of crystal grains of an oxide, whereby the shapes of these crystal grains are reflected on the surface shape of the transparent conductive oxide film 122, to form the above-mentioned texture. Therefore, the texture is already observed on the surface 122a of the transparent conductive oxide film 122 upon formation thereof. The degree of the texture that is formed on the surface 122a is strongly dependent on film forming conditions for the transparent conductive oxide film 122.
This texture has an important role on the light absorbing function of the photovoltaic device. Namely, an optical path of the light 125, which is perpendicularly incident upon the substrate 121 having a flat surface, for example, is bent along the texture formed on the surface 122a of the transparent conductive oxide film 122 in such a manner that the light 125 passes through the photoelectric conversion layer 123 obliquely to the direction of its thickness. Thus, it is possible to effectively increase the optical length as compared with incident light that passes through the photoelectric conversion layer 123 in parallel with the direction of its thickness, thereby increasing the quantity of light absorption in the photoelectric conversion layer 123.
Such an increase of the light absorption quantity caused by the texture formed on the surface of the transparent conductive oxide film is not restricted to the photovoltaic device having the structure shown in FIG. 12, but is also attained in a photovoltaic device comprising a substrate, a photoelectric conversion layer formed on the substrate and a transparent conductive oxide film formed on the photoelectric conversion layer for receiving light from the transparent conductive oxide film and photoelectrically converting the same.
Such a photovoltaic device is described in detail in Japanese Patent Laying-Open No. 61-96775 (1986) or 61-241983 (1986), for example.
In order to improve the photoelectric conversion efficiency, which is an index indicating total characteristics of a photovoltaic device, a transparent conductive oxide film must satisfy the following three requirements. First, the film must have high transmittance, in order to transmit incident light toward a photoelectric conversion layer with no loss. Second, the film must have the minimum resistance, in order to serve as an electrode for extracting optical carriers. Third, the film must have a proper shape of texture on its surface, in order to effectively bend the optical path of incident light as described above.
In general, however, it has been extremely difficult to obtain a transparent conductive oxide film that satisfies these requirements. This is particularly true because the conditions for forming the transparent conductive oxide film are related to these three characteristics by a complicated relationship.